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Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention

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On Wed, Aug 26, 2015 at 10:53:35AM -0700, Linus Torvalds wrote:
> On Tue, Aug 25, 2015 at 8:17 PM, Jason Low <jason.low2@hp.com> wrote:
> >
> > This patch addresses this by having the thread_group_cputimer structure
> > maintain a boolean to signify when a thread in the group is already
> > checking for process wide timers, and adds extra logic in the fastpath
> > to check the boolean.
> 
> It is not at all obvious why the unlocked read of that variable is
> safe, and why there is no race with another thread just about to end
> its check_process_timers().

The risk is when a next timer is going to expire soon after we relaxed
the "checking" variable due to a recent expiration. The thread which
expires the next timer may still see a stale value on the "checking"
state and therefore delay the timer firing until the new value is seen.
So the worst that can happen is that the timer firing gets delayed for
X jiffies (I guess in practice it's only 1 jiffy).

That said, posix cpu timers already suffer such race because
sig->cputimer.running itself is checked outside the sighand lock anyway.

> I can well imagine that this is all perfectly safe and fine, but I'd
> really like to see that comment about _why_ that's the case, and why a
> completely unlocked access without even memory barriers is fine.

Agreed, there should be a comment about that in the code (that is already full
of undocumented subtleties).
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[PATCH 0/3] timer: Improve itimers scalability Jason Low <jason.low2@hp.com> - 2015-08-26 05:20 +0200
  [PATCH 2/3] timer: Check thread timers only when there are active thread timers Jason Low <jason.low2@hp.com> - 2015-08-26 05:20 +0200
  [PATCH 1/3] timer: Optimize fastpath_timer_check() Jason Low <jason.low2@hp.com> - 2015-08-26 05:20 +0200
    Re: [PATCH 1/3] timer: Optimize fastpath_timer_check() Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 00:00 +0200
    Re: [PATCH 1/3] timer: Optimize fastpath_timer_check() Davidlohr Bueso <dave@stgolabs.net> - 2015-08-31 17:20 +0200
      Re: [PATCH 1/3] timer: Optimize fastpath_timer_check() Jason Low <jason.low2@hp.com> - 2015-08-31 21:50 +0200
  [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Jason Low <jason.low2@hp.com> - 2015-08-26 05:20 +0200
    Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-26 20:00 +0200
      Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 00:40 +0200
        Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Jason Low <jason.low2@hp.com> - 2015-08-27 01:00 +0200
    Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 01:00 +0200
      Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Jason Low <jason.low2@hp.com> - 2015-08-27 01:40 +0200
        Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Jason Low <jason.low2@hp.com> - 2015-08-27 07:00 +0200
        Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 15:00 +0200
          Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Jason Low <jason.low2@hp.com> - 2015-08-27 22:40 +0200
            Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 23:20 +0200
  Re: [PATCH 0/3] timer: Improve itimers scalability Andrew Morton <akpm@linux-foundation.org> - 2015-08-26 05:30 +0200
    Re: [PATCH 0/3] timer: Improve itimers scalability Jason Low <jason.low2@hp.com> - 2015-08-26 18:40 +0200
      Re: [PATCH 0/3] timer: Improve itimers scalability Oleg Nesterov <oleg@redhat.com> - 2015-08-26 19:20 +0200
        Re: [PATCH 0/3] timer: Improve itimers scalability Jason Low <jason.low2@hp.com> - 2015-08-27 00:10 +0200
          Re: [PATCH 0/3] timer: Improve itimers scalability Hideaki Kimura <hideaki.kimura@hpe.com> - 2015-08-27 01:20 +0200
            Re: [PATCH 0/3] timer: Improve itimers scalability Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 01:20 +0200
              Re: [PATCH 0/3] timer: Improve itimers scalability Hideaki Kimura <hideaki.kimura@hpe.com> - 2015-08-27 02:00 +0200
                Re: [PATCH 0/3] timer: Improve itimers scalability Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 15:20 +0200
                Re: [PATCH 0/3] timer: Improve itimers scalability Steven Rostedt <rostedt@goodmis.org> - 2015-08-27 16:50 +0200
                Re: [PATCH 0/3] timer: Improve itimers scalability Thomas Gleixner <tglx@linutronix.de> - 2015-08-27 17:20 +0200
                Re: [PATCH 0/3] timer: Improve itimers scalability Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-27 17:20 +0200

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